# A Tight Composition Theorem for the Randomized Query Complexity of Partial Functions

Shalev Ben-David,
Eric Blais

November 2020

### Abstract

We prove two new results about the randomized query complexity of composed functions. First, we show that the randomized composition conjecture is false: there are families of partial Boolean functions f and g such that R(f∘g)≪R(f)R(g). In fact, we show that the left hand side can be polynomially smaller than the right hand side (though in our construction, both sides are polylogarithmic in the input size of f).

Second, we show that for all f and g, R(f∘g)=Ω(noisyR(f)⋅R(g)), where noisyR(f) is a measure describing the cost of computing f on noisy oracle inputs. We show that this composition theorem is the strongest possible of its type: for any measure M(⋅) satisfying R(f∘g)=Ω(M(f)R(g)) for all f and g, it must hold that noisyR(f)=Ω(M(f)) for all f. We also give a clean characterization of the measure noisyR(f): it satisfies noisyR(f)=Θ(R(f∘gapmaj_n)/R(gapmaj_n)), where n is the input size of f and gapmaj_n is the √n-gap majority function on n bits.

Publication

*Proceedings of the 61st Annual IEEE Symposium on Foundations of Computer Science (FOCS)*